Field of the Invention
The present invention concerns a method for echo planar magnetic resonance imaging wherein raw data are acquired (entered into k-space) with a zigzag-type trajectory, as well as a magnetic resonance apparatus for implementing such a method. In particular, the invention concerns applying a certain series of phase encoding gradient fields, which enable the zigzag-type trajectory. The invention also concerns techniques for parallel imaging employing the zigzag-type trajectory.
Description of the Prior Art
Magnetic resonance (MR) tomography is an imaging modality that enables acquisition of two-dimensional or three-dimensional image data sets that allow structures in the interior of a patient to be shown in an image with a spatial resolution. MR data are acquired by operating an MR data acquisition unit magnetic to align magnetic moments of protons in a patient in a basic magnetic field or main magnetic field (B0), such that a macroscopic magnetization aligns along a longitudinal direction. This longitudinal magnetization is subsequently deflected from the rest position parallel to the basic magnetic field by irradiating with radio-frequency (RF) pulses (excitation, TX). A transverse magnetization is thereby created. For irradiation of the RF pulses typically dedicated RF transmitter coils of the MR apparatus are employed.
The relaxation of the transverse magnetization back to the rest position, namely the magnetization dynamics, is subsequently detected by employing one or more RF receiver coils of the MR apparatus (imaging, RX). Here a spatial coding of the detected MR data is achieved by applying various magnetic field gradients (for slice selection, phase encoding or frequency encoding). Raw data are acquired in the spatial frequency domain (k-space). By means of a Fourier transformation of the raw data, a MR image in time image or time domain can be obtained. Excitation and imaging are executed as part of a so-called MR measurement sequence.
For imaging, various measurement sequences can be employed. A well-known measurement sequence is the so-called echo planar MR imaging sequence (echo planar imaging, EPI). A tailored dephasing and rephasing of the transverse magnetization for obtaining a so-called gradient echo sequence can occur by applying of gradient fields. For example, by alternation of readout gradient fields of opposite signs, as part of EPI a number of pairs of gradient echoes (gradient echo sequence) can be generated and thereby, by interacting with phase encoding gradient fields, k-space can be sampled (filled at data entry points) for obtaining the raw data.
EPI is in particular employed when fast imaging is desired. Specifically, fast imaging can relate to the time required for obtaining the raw data being relatively short. Aside of this, EPI finds broad application both in clinical fields as well as in neurosciences. Diffusion weighted MRI, which can be used to study nerve links in the brain, and functional imaging for measurement of activation in the brain, are notable examples of the use of EPI.
It is possible for the EPI image quality to be relatively sensitive with respect to so-called susceptibility artifacts and T2* relaxation. Susceptibility artifacts can lead to image distortions that depend, inter alia, on the time between the various echoes of a gradient echo sequence (echo spacing). Further, the T2* relaxation can limit the possible resolution. During readout, the MR signal is typically reduced due to T2* relaxation, i.e., the longer a gradient echo sequence, the stronger the impact of the T2* relaxation on the achievable resolution. Besides these physical limitations, it is also possible for further technical requirements of MR apparatuses and EPI imaging to be comparably demanding. For example, typical EPI sequences impose high demands on the MR apparatus, because a comparably large number of gradient fields are rapidly switched. Mechanical vibrations caused by this can lead to strong noise generation, which not only reduces patient comfort, but also requires a more sophisticated protection for the hearing of the patient (two-fold hearing protection by ear-phones and ear plugs). Further, strong mechanical vibrations can lead to increased wear on the gradient coil and the supply line thereof, which causes increased maintenance costs or even damage to the MR apparatus.
Various techniques are known to reduce challenges and limitations of EPI measurement sequences that are caused by susceptibility effects and T2* relaxation.
For example, by employing certain sampling schemes of k-space, i.e. by employing certain k-space trajectories, an efficient sampling of k-space (acquiring of raw data) can be achieved. It is typical for EPI measurement sequences that the entirety of k-space is read out (filled) after a single excitation of the transverse magnetization. In this respect it can be advantageous to acquire raw data along particularly short k-space trajectories. A shorter k-space trajectory typically results in a shorter gradient echo sequence, i.e. in a reduced T2* relaxation effect.
A further starting point for the reduction of the challenges mentioned above is the use of gradient systems that are configured to create relatively strong gradient fields, i.e., having large amplitudes. Further, the gradient systems can be configured to switch the gradient fields faster, i.e., with a higher slew rate. The time required to switch the gradient fields can thereby be shortened, so the readout time of the EPI sequence can be shortened as well. It can then also be possible to reduce the total duration of the measurement. The total duration also takes into account the readout time and additionally the time required for excitation and potential times of waiting. However, employing of faster and stronger gradient systems is limited by the technological and physiological framework. For example, from a technological point of view, by employing faster and stronger gradient systems, the mechanical stress, e.g., vibrations, and the required amplifier power can increase significantly and reach the limits of technical feasibility. From a physiological point of view, the peripheral nerve stimulation of the patient limits the extent of switching faster and stronger gradient fields.
Further, various techniques known as partial parallel acquisition (PPA) are available, which can be employed as part of the EPI measurement sequence to reduce both the gradient echo sequence as well as the echo spacing and to enhance EPI imaging quality. Such techniques rely on an undersampling of k-space, i.e., the amount entered into k-space for example, certain raw data, k-space rows or k-space points can be left unfilled. By reduction of the amount of raw data, the gradient echo sequence can be shortened (increased resolution) and by skipping data, which are reconstructed later, the effective echo spacing can be reduced by the acceleration factor of the PPA-technique (reduced distortions). Examples for such techniques are Generalized Auto-Calibrating Partial Parallel Acquisition (GRAPPA), Sensitivity Encoding (SENSE), Simultaneous Acquisition of Spatial Harmonics (SMASH) imaging techniques and Sensitivity Profiles from an Array of Coils for Encoding and Reconstruction in Parallel (SPACE RIP).
The method employed most commonly at the moment, GRAPPA, brings about the advantage of a self-calibrating method and operating on completely clean raw data, (k-space data). In GRAPPA, an inversion of a relatively small matrix for determining the reconstruction parameters (reconstruction kernel) is required. The reconstruction kernel needs to be adapted to a particular sampling scheme (with which k-space is undersampled). The distance between data points that are used for the reconstruction (source points) and the data points to be reconstructed (target points) needs to be fixed. This is typically the case for so-called Cartesian trajectories (equally spaced distance). With a zigzag-type trajectory for acquisition of the raw data, the distances between neighboring data points are not constant anymore. Therefore, a direct application of PPA techniques such as GRAPPA is not possible or only possible to a limited degree. More sophisticated non-Cartesian PPA techniques, due to their increased complexity, such techniques have not been adopted for regular clinical use.
Therefore, a need exists for optimized techniques of EPI imaging. In particular, a need exists for such techniques that achieve a shortened readout time and a shortened echo spacing, whereby the image quality can be enhanced. Further, a need exists for such techniques with limited technological and physiological stress due to gradient fields.